zashiki-warashi
From Obakemono:
"A household spirit resembling a child of either gender from about three to twelve years of age. The name zashiki-warashi comes from the interior of Iwate Prefecture, but similar legends are found all over Japan. It is often said that when this being dwells in a house, its family will be prosperous, but when it leaves, the house will fall into ruin, and also that the zashiki-warashi is never seen, save for just as it is about to leave. Its presence however, is often marked by the mischief it does to sleeping people, such as moving pillows about and pulling on bedding."